Curable silicone compositions are applied to paper substrates to aid in the release of adhesive materials thereon. Laminates comprising a release paper coated with a pressure sensitive adhesive and a sheet material that can be a decorative lamina or label are used by stripping off the release liner which is discarded and affixing the lamina or label onto a surface.
Typically these release compositions cure by one of two mechanisms, thermal curing or photo-catalytic curing. Thermally curing paper release systems generally are comprised of the following compositions:
(A) a linear alkenyl substituted polysiloxane polymer that is the primary component or base polymer of the curable composition;
(B) an hydrosilylation addition cure catalysts, typically either a platinum or rhodium based catalyst;
(C) a cure inhibiting compound or mixtures thereof to increase the useful life of the coating bath; and
(D) a hydride functional cross-linking silicone, typically a methyl hydrogen siloxane polymer, copolymer or oligomer.
While the general practice usually employs linear base polymers, (A), solventless, high solids content formulations have been described. As described in U.S. Pat. No. 4,448,815 ('815) a linear alkenyl siloxane base copolymer is a copolymer of:
(1) EQU R.sub.c R.sub.d.sup.1 Si.sub.(4-c-d)/2 1)
where R is generally an alkyl radical, R.sup.1 is a low molecular weight olefinic substituent such as vinyl or allyl, c has value from 0 to 2 and the average of value of the sum c+d is 0.8 to 3; and
(2) EQU R.sub.n SiO.sub.(4-n)/2 2)
where R is generally an alkyl radical and n has a value of 0.8 to 2.5. The preferred base copolymer of the '815 patent has the following linear structure: EQU (H.sub.2 C.dbd.CH)R.sub.2 Si--O--(R.sub.2 Si--O--).sub.i --(RR.sup.1 Si--O--).sub.j --SiR.sub.2 (H.sub.2 C.dbd.CH)
where the subscripts i and j are integers.
U.S. Pat. No.4,774,111 ('111) describes a variation of the above linear copolymer where the R group in formula 2 is selected from alkyl and alkenyl radicals. The polymer of the '111 patent is defined as being substantially linear, i.e. having no more than a trace amount of T or Q groups. This substantially linear requirement for alkenyl functional heat curing silicone release compositions is repeated in U.S. Pat. Nos. 4,772,515; 4,783,552 and 5,036,117.
In contrast, the possibility of branched alkenyl polymers is admitted by the structural formulas recited in U.S. Pat. No. 4,057,596 ('596). In the '596 patent the composition comprises:
(A') a substantially linear vinyl chainstopped polymer;
(B') a linear methyl hydrogen polymer;
(C') a methyl vinyl polysiloxane having at least three vinyl groups per molecule;
(D') a methyl hydrogen polysiloxane having at least three hydride hydrogen atoms per molecule; and
(E') a platinum hydrosilylation catalyst. Component (C') is described in the '596 patent as containing (H.sub.2 C.dbd.CH)(CH.sub.3)SiO.sub.2/2 (D.sup.vi), (H.sub.2 C.dbd.CH)(CH.sub.3).sub.2 SiO.sub.1/2 (M.sub.vi), and (H.sub.2 C.dbd.CH)SiO.sub.3/2 (T.sup.vi), units either singly or in combination with (CH.sub.3).sub.2 SiO.sub.2/2 (D), (CH.sub.3).sub.3 SiO.sub.1/2 (M), and (CH.sub.3)SiO.sub.3/2 (T). The optional inclusion of vinyl substituted T units and methyl T units permits the composition of the '596 patent to possess branched structures.
U.S. Pat. No. 4,386,135 describes a terminally unsaturated silicone polymer having the formula EQU R.sub.4-a Si((R.sub.2 SiO--).sub.b OSiR.sub.2 R.sup.2).sub.a 3 )
where a may be 2, 3, or 4. When a=4 the formula produces a Q resin. When a=3, a T structure results and the structure possesses only a single branch point. When a=2, the formula devolves to an alkenyl chain stopped linear polymer.
The alkenyl functional silicone polymer paper release compositions typically used fall into one of two categories:
1) a linear alkenyl chainstopped polymer: EQU M.sup.vi D.sub.x M.sup.vi 4)
where M.sup.vi indicates an alkenyl chainstopping M group or
2) multi-functional alkenyl copolymers: EQU M.sup.vi D.sub.x D.sup.vi.sub.y M.sup.vi 5)
where D.sup.vi indicates an alkenyl substituted D group. The alkenyl chain stopped polymers, M.sup.vi D.sub.x M.sup.v, generally cure faster than the multi-functional copolymers, M.sup.vi D.sub.x D.sup.vi.sub.y M.sup.vi. As paper release composites are delaminated, the formulations based on the linear alkenyl chainstopped polymers show significant increases in the delamination force necessary as delamination speed increases. In contrast, while the multi-functional alkenyl polymers tend to have a slower curing speed the increase in delamination force with increasing delamination speed is not nearly as great proportionately.
Accordingly, it is desirable to provide a composition having the high cure speed of the alkenyl chainstopped polymers with the lower release force characteristics of the multi-functional alkenyl compositions. It would also be desirable to have the release force change as little as possible as a function of increasing delamination speed.